This invention relates to control arrangements for adjustable pumps. In particular it relates to such control arrangements where it is desired that the product of the lifting pressure times the flow be kept constant, independent of the driving speed of the pump. A throttle valve is arranged in the conduit communicating with the output side of the pump. Further, the pump has a regulating element which is acted upon by two pressure-loaded pistons.
This type of regulating system is also known as a power control system and the product of lifting pressure times quantity of liquid pumped is held constant by arranging a lever on which acts a control spring from the one side while the lifting pressure is applied to the other side by means of a piston. The lever then controls a piston which in turn controls the regulating element of the pump. For increasing lifting pressures, the quantity of liquid is decreased and vice versa. An exact hyperbolic characteristic curve can be created with such a control system, but only the starting torque of the pump is held constant while the delivered hydraulic power may vary in dependence with the speed of the pump.
Other regulating systems are known which do manage to maintain the delivered hydraulic power constant. Here a spring and a piston interact with a lever, the spring being adjusted to represent the desired power output, while the piston delivers a pressure corresponding to the lifting pressure of the pump. Of course a mechanical root extracting element must be furnished in order to equalize the two dimensionally unequal quantities. This type of regulating system is therefore very complicated, expensive and subject to malfunction.